As known, Read Only Memory (ROM) or Programmable ROM (PROM) devices (referred to collectively herein as ROM) allow only reading of the information stored in the device. Writing only occurs once, when the information is initially programmed into ROM. The ROM is non-volatile. Once written, the stored information is permanently resident. It is preserved through power outages, and is unaffected by power up/power down sequences. For this reason ROMs are used to store permanent data (i.e. not subject to change) or program instruction sets. If the data or instruction set must change, the ROM must be replaced with a newly programed ROM.
More recently Electrically Eraseable, Programable ROM (EEPROM) devices have been used to replace ROMs, and also Random Access Memory (RAM) devices, in user systems. The EEPROM allows writing to ("re-programing" of) the device while installed in a system. A number of EEPROM manufacturers produce models designed to be pin-for-pin compatible with both ROM and RAM devices, in the same pin-pad layout, allowing for use of either type in a given application. In order to plug an EEPROM into a ROM socket, however, some type of write protection must be added to the system to protect the EEPROM against inadvertent "writes" during system power up/power down sequences. Therefore, field replacement of installed system ROMs with EEPROMs is difficult, or impossible.